The present invention relates to release coatings, and in particular to release coatings which can be used in conjunction with pressure-sensitive adhesives.
Pressure sensitive adhesives (PSAs) have widespread uses a bonding agents for manufacturing tapes, labels, decals, and the like. PSAs adhere to many surfaces with no more than applied finger pressure, e.g., various tapes (office, packaging, masking, etc.) or labels (office, shipping, identification, etc.). The adhesive is typically coated onto a xe2x80x9cbacking,xe2x80x9d which includes any suitable substrate, such as films, foil, and paper. The adhesive may be coated onto one or both sides of the backing. The side of the backing coated with adhesive is referred to as the xe2x80x9cadhesive-coated backing side.xe2x80x9d
In the case of tapes, the adhesive-coated backing is typically wound onto itself to form a roll, wherein the adhesive-coated backing side is in direct contact with the adhesive-free backing side of the continuous backing in the roll. As a result, tapes typically have two sides or surfaces: one side that is coated with a PSA, and the other side of the tape, which is free of adhesive. It is, therefore advantageous for the adhesive layer to have a different affinity for one side of the tape than that of the opposite side (e.g., for the two sides) of the tape. Such differential affinity is typically achieved by two methods: the primer coating method and the release coating method. In the primer coating method, an adhesive that has a low affinity for the backing is selected, and a primer coat is used to make the adhesive layer bond strongly to the backing. Upon rolling, the adhesive-coated backing side adheres therefore less strongly to the adhesive-free backing side. Non-limiting examples of this method include solvent-based natural rubber adhesives coated onto unplasticized polyvinylchloride film and aqueous-based acrylic adhesives coated onto oriented polypropylene film, wherein the coated backing side has been xe2x80x9cprimedxe2x80x9d by corona discharge treatment.
In the release coating method, a release coating is applied to the adhesive-free side of the tape and an adhesive is applied to the other side of the tape to form the adhesive-coated backing side. The release coating has a strong affinity for the backing but a weak affinity for the adhesive. The adhesive-free side of the backing, which is coated with a releasing agent, is referred to herein as the xe2x80x9crelease-coated backing side.xe2x80x9d In certain cases, both sides of the backing may be coated with adhesive and a separate backing containing the release-coating (release-coated paper) may be applied to one or both sides of the adhesive-coated backing sides. A backing having both sides coated with adhesive is referred to herein as xe2x80x9cdouble adhesive-coated backingxe2x80x9d, and the side or sides which contain the release-coated paper is referred to herein as the xe2x80x9crelease-coated paper side.xe2x80x9d Such release coatings make it possible for the tape to be unwound from the roll (i) without using excessive force (low release peel force) and (ii) without causing the adhesive to adhere to the release-coated backing side.
Release coatings should also have the following characteristics. The low affinity of the release coating toward the adhesive should be maintained over prolonged exposure to wide variations of temperature and humidity. The release coating should also strongly bond to the backing side so that it does not delaminate when aggressive adhesives are used. Lastly, it is desired that components of the release coating do not migrate into the adhesive, thereby deteriorating the properties of the adhesive.
Conventional release coatings have been based on various polymers and copolymers prepared from monomers, such as alkylacrylates and alkylmethacrylates, vinylacetate, vinylchloride and acrylonitrile and combinations thereof. These monomers, however, are relatively expensive, and some of these monomers also require special and more expensive handling because of their toxicity. In addition, such conventional coatings often fail when the rolled tape is aged under heat and humidity.
It is, therefore, an object of the present invention to provide release coating compositions that provide release coatings with acceptable peel force values after prolonged exposure to heat and humidity. It is another object of the present invention to provide release coating compositions that include relatively inexpensive polymer components. These and other objects of this invention will become apparent in light of the following disclosure.
The present invention is directed to a release coating composition that includes (a) from about 20% to about 55% by weight of a copolymer having a Tg from 0xc2x0 C. to about 75xc2x0 C., and (b) from about 45% to about 80% by weight of solvent. The copolymer has from 0% to about 50% by weight of acrylonitrile monomer, and from about 50% to about 100% by weight of butadiene and styrene monomers. The copolymer can further include one or more stability enhancing monomers. In one embodiment, the release coating composition can further include a release promoting additive. The present invention is also directed to release coatings obtained from applying and drying the release coating composition.
In another embodiment, the present invention is directed to a release coating that includes (a) from about 80% to 100% by weight of a copolymer having a Tg from 0xc2x0 C. to about 75xc2x0 C., and (b) from 0% to about 20% by weight of a release promoting additive. The copolymer has from about 0% to about 50% by weight of acrylonitrile monomer, and from about 50% to about 100% by weight of butadiene and styrene monomers.
In still another embodiment, the present invention is directed to an article of manufacture, which includes (a) a backing having an adhesive-coated backing side, and (b) a release coating applied to the other side of the backing of (a) or a separate backing. The release coating is situated so that it is in contact with the adhesive-coated backing side. The release coating has from about 80% to about 100% by weight of a copolymer having a Tg from 0xc2x0 C. to about 75xc2x0 C., wherein the copolymer comprising from 0% to about 50% by weight of acrylonitrile monomer, and from about 50% to about 100% by weight of butadiene and styrene monomers. The release coating of this article of manufacture can further include a release promoting additive.
It has been surprisingly found that release coatings prepared from polymers of butadiene, styrene and acrylonitrile (BSA polymer) provide excellent, low release force even after prolonged exposure to heat and humidity. Until now it has been believed by those skilled in the adhesive/coatings arts that polymers of styrene and butadiene could not be successfully modified to form effective release coatings. This belief results from the well accepted chemical phenomenon that like materials tend to have a high affinity towards like materials. Since pressure-sensitive adhesives are typically produced from hydrocarbon based polymers of styrene and butadiene, e.g., triblock styrene-butadiene-styrene polymers, it was believed that effective release coatings could not contain styrene and butadiene monomers.
In line with this belief, conventional release coatings have contained various polymers, prepared from monomers which chemically differ from styrene and butadiene, such as alkylacrylates and alkylmethacrylates, vinylacetate, vinylchloride, acrylonitrile, and combinations thereof. Although effective, these conventional polymers used in release coatings are relatively expensive in comparison to the BSA polymers used in the present invention, and some also require special and more expensive handling because of their toxicity.
A. Release Coating Compositions
The release coating compositions of the present invention include: (i) a polymer of butadiene, styrene, and acrylonitrile, and (ii) a solvent. Preferably, the release coating compositions of the present invention further include one or more release promoting additives.
The release coating compositions of the present invention include from about 20% to about 55%, preferably from about 30% to about 40% by weight of a polymer of butadiene, styrene and acrylonitrile (xe2x80x9cBSA polymerxe2x80x9d), from about 45% to about 80%, preferably from about 60% to about 70% by weight of a solvent. The term xe2x80x9cabout,xe2x80x9d as used herein, means plus or minus 10% of the stated value. The terms xe2x80x9cBSA polymerxe2x80x9d and xe2x80x9ccopolymer,xe2x80x9d as used herein, mean a polymer having two (styrene and butadiene), or more different monomers.
The solvent can be any suitable fluid that can be evaporated at room temperature or upon applying heat. Preferred solvents those known in the art for use in emulsion polymerization, mass polymerization, and mass-suspension polymerization. More preferred are solvents used in emulsion polymerization. Nonlimiting examples of preferred solvents include water, methanol, isopropanol, and mixtures thereof.
These BSA polymers have a Tg from 0xc2x0 C. to about 75xc2x0 C., preferably from about 20xc2x0 C. to about 60xc2x0 C. The term xe2x80x9cTg,xe2x80x9d as used herein, means the glass-transition temperature of the polymer as determined by the Fox equation, which is described in Sperling, Polymeric Multicomponent Materials, Chapter 1, pp. 24-29 (1997). The BSA polymers according to the present invention have from about 0% to about 50%, preferably from about 5% to about 50% by weight of acrylonitrile monomer, and from about 50% to about 100%, preferably from about 50% to about 95% by weight of butadiene and styrene monomers. More preferably, the BSA polymers have from about 15% to about 40% by weight of acrylonitrile monomer and from about 60% to about 85% by weight of butadiene and styrene monomers. Typically, the BSA polymers contain (i) from about 10% to about 35%, preferably from about 15% to about 25% by weight of butadiene monomer, and (ii) from about 25% to about 75%, preferably from about 45% to about 55% by weight of styrene monomer.
The BSA polymers according to the present invention can be manufactured according to any polymerization process known in the art. Such polymerization processes include, but are not limited to, emulsion polymerization, mass polymerization, and mass-suspension polymerization, as described in Kirk-Othmer, Encyclopedia of Chemical Technology, 4th Ed. Vol. 1, pp. 396-398. Preferably, the BSA polymers are manufactured by emulsion polymerization process.
In one embodiment, the BSA polymers of the present invention further include from about 0.50% to about 15%, preferably from about 1.0% to about 10% of at least one stability enhancing monomers. The term xe2x80x9cstability enhancing monomers,xe2x80x9d as used herein, means a monomer that provides colloidal stability for the polymer during emulsion polymerization and/or improves polymer properties, such as tensile strength. Examples of stability enhancing monomers include, but are not limited to, acrylic acid; itaconic acid; acrylamide, such as N-methylolacrylamide; fumaric acid; methacrylic acid; maleic acid; and the like. Most preferably, the BSA polymers of the present invention include from about 0.5% to about 3.0% by weight of acrylic acid monomer, from 0% to about 1.5% by weight of itaconic acid monomer, and from 0.5% to about 3.0% by weight of acrylamide monomer.
In another embodiment, the release coating compositions of the present invention further include from about 1% to about 15% by weight of a release promoting additive, which is post-added. Post-added means that the additive is added after polymerization of the polymer is completed. Without wanting to be limited by any one theory, it is believed that these release promoting additives help to maintain or enhance the release properties of the release coating. Examples of release promoting additives useful herein include, but are not limited to, anionic surface active agents, cationic surface active agents, zwitter-ionic surface active agents, non-ionic surface active agents, and mixtures thereof. Typically, these surface active agents contain C8-C22 alkyl chains, C8-C22 alkyl substituted aryls, silicon atoms, fluorine atoms, and combinations thereof, as described in xe2x80x9cHandbook of Pressure-Sensitive Adhesive Technology,xe2x80x9d ed. D. Satas, Satas and Associates, Warwick, R.I., 1999, Chap. 26, pps. 637-646. Non-limiting examples of useful release promoting additives include N-alkyl sulfosuccinamates, dialkyl sulfosuccinates, alkyl sulfates and sulfonates, alkyl-polyalkyleneoxidesulfates, alkyl-aryl-polyalkyleneoxidesulfates, alkyl-aryl-sulfonates, alkylpolyalkyleneoxides, alkyl-aryl-polyalkyleneoxides, block copolymers of alkyloxides, and mixtures thereof.
Specific examples of release promoting additives useful herein include, but are not limited to, Aerosol A-22 (manufactured by Cytec Industries Inc., located in West Paterson, N.J.), Iconol OP40 and Pluronic 62 (both manufactured by BASF Corporation, Mount Olive, N.J.), Ammonyx SO (manufactured by Stepan Company, Northfield, Ill.) and mixtures thereof.
Optional ingredients: The release coating compositions of the present invention can further include polymers that are known and conventionally used in release coatings. The release coating composition can further include from 0% to about 95%, preferably from 0% to about 50% by weight of a conventional release coating homopolymer and/or copolymer prepared from monomers selected from the group consisting of alkylacrylates and alkylmethacrylates, vinylacetates, vinylchlorides, acrylonitriles, siloxanes, organo-siloxanes, and combinations thereof. Typically, the BSA polymers of the present invention would be mixed with such conventional release coating polymers to form a blend of the polymers.
B. Articles of Manufacture Having Release Coatings
In another embodiment, the present invention is directed to an article of manufacture including a backing having an adhesive-coated backing side and a release-coated backing side, as described above. In this embodiment, the backing is wound onto itself so that the adhesive-coated backing side is placed in physical contact with the release-coated backing side. The dry release coating includes from about 80% to about 100% by weight of the BSA polymer. The dry release coating can further include up to about 20% by weight of a release promoting additive.
In an alternative embodiment, one or both sides of the backing can be coated with adhesive (double adhesive-coated backing), and a release-coated paper having a release coating, as described above, on a separate backing is placed in physical contact with one or both sides of the adhesive-coated backing sides.
Pressure sensitive adhesives that are useful according to the present invention are conventional materials typically used and known in the art. Examples of pressure sensitive adhesives include, but are not limited to, natural rubber adhesives; butyl rubber adhesives; vinyl ether adhesives; acrylic adhesives; solvent-based rubber adhesives including cis-1,4-polyisoprene; block copolymer adhesives including styrene-butadiene-styrene block and styrene-isoprene-styrene block copolymers; hot-melt block copolymer adhesives; synthetic rubbers including styrene-butadiene rubber; silicone based pressure sensitive adhesives; and the like.
Backings that are useful according to the present invention are any suitable substrates known in the art. Examples of useful backings include, but are not limited to, films, foil, paper, nonwovens, wovens and composites thereof.
The release coating and the adhesive coating can be applied to the backing using conventional methods known in the art, such as knife coating, roller coating and print coating. After the coatings are applied to the backing, they may be dried by any heating method that is well known in the art. Typically, the dry thickness of the release coatings are from about 0.1 to about 0.5 mils according to the selection of backing, pressure-sensitive adhesive, and coating technique. Dry release coating thicknesses from about 0.2 to about 0.35 mils are preferred. For the BSA polymers of the present invention, 0.39 mils is equal to about 10 g/m2.